Technical Abstract:
Internal parasitic nematodes impact the livestock industry through losses in reproductive efficiency, rate of gain, carcass quality, milk production, or immune response. The frequent use of anthelmintics with drug formulations in which chemical activity persists for long periods selects for worm resistance and has contributed to parasite resistance to multiple classes of dewormers. We evaluated the effectiveness of two classes of anthelmintics, avermectin and fenbendazole, on fecal parasite load in yearling heifers grazing on irrigated pastures in south central Nebraska. Both of these anthelmintics have been used historically at U.S. MARC. Hence, a reduction in efficacy for these two historically used anthelmintic classes would be indicative of resistance. Spring-born heifers (n=1,015, average BW=295 kg) were grazed on irrigated, mixed legume and cool season grass irrigated paddocks starting 15 April, 2013. Heifers were managed in four pastures and fecal egg counts (FEC) 10% of each grazing group (25 hd minimum) were monitored weekly using a modified Wisconsin fecal floatation technique. When average FEC numbers for the sample reached >25 eggs per 2 grams, heifers in the group were weighed, fecal sampled for FEC (pre-treatment), and randomly assigned to one of three treatment groups (no dewormer; Avermax injectable (avermectin) at 1 ml/50kg BW subcutaneously; or Safeguard 10% Suspension (fenbendazole) at 2.3 ml/45.5 kg BW orally) by age and breed stratification. Treatments occurred 16, 17, 26, and 27 July, 2013. Animals were sampled and FEC counts were obtained two weeks later (post-treatment). The percentage of zeros for FEC was 4.5 for pre-treatment, and 39.0 for post-treatment. Because of the high proportion of zero valued FEC, pre-treatment and post-treatment FEC were analyzed assuming a zero inflated negative binomial distribution. The zero inflated negative binomial model fit better than the standard negative binomial for post-treatment FEC (P=0.0026) but not for pre-treatment FEC (P=0.363). The effects of pasture (or breed) and treatment interacted (P=4.01x10**-21**) with treatment having larger effect in pastures with higher FEC pre-treatment. Considering main effects, pasture affected FEC both pre- (P=3.27x10**-19**) and post-treatment (P=1.13x10**-13**). Treating animals with either Avermax or Safeguard reduced FEC (P=4.7910**-96**) and Safeguard’s effect was greater than Avermax (P=1.03x10**-13**). Treatment with anthelmintics is efficacious and has greater impact when parasitic load is greater. High efficacy of treatment indicates that little or no resistance has accumulated in parasitic nematode populations at USMARC.
KEY WORDS: Parasitic intestinal nematodes, cattle, fecal egg counts, resistance, althelmintics